There are various mobile communications systems, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, PCS (personal communications service) widely used in the U.S., and PDC (personal digital cellular system) used in Japan. Small, lightweight, high-frequency circuit devices adapted to pluralities of systems include, for instance, dual-band, high-frequency switch modules adapted to two systems of EGSM and DCS, triple-band, high-frequency switch modules adapted to three systems of EGSM, DCS and PCS, etc. Wireless LAN data communications according to the IEEE802.11 standard, etc. are also widely used at present. The wireless LAN standard includes pluralities of standards having different frequency bands, etc., and various high-frequency circuits are used in wireless-LAN, multiband communications apparatuses.
To output signals having relatively large power, a transmission system of a cell phone comprises a high-power amplifier (high-frequency amplifier) of several W. To provide small cell phones, etc. with low power consumption, a high-power amplifier consuming most of DC power is required to have high DC-RF power-converting efficiency, which may also be called power-adding efficiency, and a small size. In a high-frequency device comprising an antenna switch module and a high-power amplifier in combination, which is used in mobile communications apparatuses such as cell phones, etc., its high-frequency circuit is provided with an output-matching circuit for impedance matching. Accordingly, for the purpose of miniaturization, not only the high-power amplifier but also the antenna switch module, the output-matching circuit, etc. should be miniaturized.
The output-matching circuit is constituted by a transmission line, and pluralities of capacitors connected to the transmission line. In the output-matching circuit of JP 2004-147166 A, a linear transmission line is formed on a surface layer of a laminate, such that impedance matching can be finely adjusted after integrating a high-frequency amplifier module with a high-frequency switch module. Because the transmission line should be sufficiently long to secure sufficient impedance, the output-matching circuit of JP 2004-147166 A is not suitable for miniaturization. In addition, a long transmission line has large conduction loss, hindering higher performance.
Harmonics contained in high-frequency power output from the high-frequency amplifier and passing through the output-matching circuit should be removed by a filter circuit, etc. However, the filter circuit does not necessarily attenuate harmonics fully, and larger attenuation makes the filter circuit more complicated and larger. It is thus difficult to achieve both the suppression of harmonics and the miniaturization of the high-frequency circuit.
In cell phone systems, to avoid the mixing of signals with nearby cell phones to keep communications quality stably, control signals (power control signals) are sent from base stations to cell phones such that transmitting outputs have the minimum power necessary for communications. An automatic power control (APC) circuit operated based on control signals acts to control the gate voltage of a high-frequency amplifier to provide a transmitting output with necessary power for communications. This control is conducted by comparing signals detecting power output from the high-frequency amplifier with the power control signals sent from the base station. The output of the high-frequency amplifier is detected, for instance, by a coupler connected to its output terminal. However, a conventional coupler mounted onto a printed circuit board discretely from an output-matching circuit, etc. occupies a large mounting area, preventing the miniaturization of mobile communications apparatuses.
JP 2003-324326 A proposes a high-frequency amplifying apparatus comprising a high-frequency amplifier, an output-matching circuit and a coupler integrally formed on a substrate. However, the formation of the output-matching circuit and the coupler on one substrate still fails to fully miniaturize the high-frequency device. In general, a main line and a sub-line in the coupler are as long as about ¼ of the wavelength of a frequency used. Because the ¼ wavelength of a band mainly used in cell phones, etc. is about 15-100 mm, the plane structure described in JP 2003-324326 A fails to materialize as small high-frequency devices as 10 mm or less on each side. Also, because there is a likelihood of short-circuiting when a main line and a sub-line in the coupler are made closer to each other on a substrate for miniaturization, the reduction of a gap between the main line and the sub-line is limited.